1. Field of the Invention
The present invention relates to electronic ballasts, and more particularly to electronic ballasts that clamp a voltage across a switch and recycle leakage energy of a transformer such that a power converter has high power conversion efficiency with no voltage spike in the switch.
2. Description of the Related Art
Traditional ballasts for use in fluorescent lamps consisted of large and heavy magnetic coils. These have been replaced by electronic ballasts that are compact and light. A common device used as an electronic ballast is a self-oscillating, push-pull converter. One well-known push-pull converter is shown in FIG. 1. As shown in FIG. 1, the converter consists of a DC power source Vin, switches S1 and S2, and transformer T. The switches S1 and S2 are each a standard metal-oxide semiconductor, field-effect transistor (MOSFET). Ds1 and Cs1 represent body diode and internal capacitance, respectively, of switch S1, and Ds2 and Cs2 represent body diode and internal capacitance, respectively, of switch S2. Transformer T contains three windings, N1, N2 and N3, and is designed such that a current present in either winding N1 or N2 produces a current in winding N3 to drive the load Ro, typically a fluorescent lamp. Inductor Lr and capacitor Cr form a standard resonant tank to provide high frequency voltage to the load Ro. Three current paths flowing in the circuit are represented as having currents is1, is2 and iLr. Voltage V0 is shown across load Ro.
FIG. 2 is a graph of the waveforms associated with the conventional push-pull electronic ballast circuit of FIG. 1. As shown in FIG. 2 and also found in other like electronic ballasts is a phenomenon of overvoltage or high voltage spikes that occur across the switches when the device is switched off For example, at to when S1 is switched off a spike is shown in FIG. 2 to occur in the voltage across S1, i.e. Vds1. The principle reason for the voltage spike is that the opening of switch S1 creates an abrupt interruption (discontinuity) in the current through N1. There is an release of stored energy due to a leakage inductance associated with winding N1 (represented as Lk1 in FIG. 1), thereby inducing a current that charges C,s1 and consequently creating a high voltage spike across switch S1.
The same voltage spike characteristic occurs in the voltage across S2, i.e. Vds2, at time t2 when S2 is switched off. The voltage spike is caused by transient or leakage inductance Lk2 associated with transformer winding N2. The circuit of FIG. 1 provides no avenue for the released energy of the winding to flow other than the internal capacitance of the adjacent switch, which results in the voltage spike. In the prior art the leakage inductances are always present to some extent and there is always a danger of inducing these high voltage transients when switching off. The faster the switching the greater the voltage spikes. An excessive voltage spike will result in permanent damage to the switching device such as a burn through of the semiconductor layers.
One method of reducing the voltage spikes is to include a xe2x80x9csnubberxe2x80x9d circuit comprised of an additional diode, capacitor and resistor, connected in parallel with the switch (such as S1 and S2). While the snubber circuit can limit the peak voltage of the spike, it slows down the effective switching speed of the circuit, and in doing so it absorbs energy that is dissipated as heat, thus reducing the overall power conversion efficiency of the ballast.
Another method of reducing the voltage spikes is to xe2x80x9cclampxe2x80x9d the voltage across the switch (such as S1 and S2). This is accomplished by two conventional methods. A first method connects a zener diode across the switch. As the voltage spike occurs the Zener diode turns on allowing the current to flow through, thus reducing or eliminating the voltage spike across the switch. A second method connects a diode in series with a with a parallel capacitor and resistor network across the switch. The capacitor charges to a constant voltage through a current flow, thus absorbing the voltage spikes. The resistor dissipates the stray inductances while maintaining the voltage across the capacitor. As with any clamping circuit, stray circuit inductances exist, and voltage spikes will be produced. Also, a diode by its nature does not provide an instantaneous clamping action.
It is therefore an aspect of the present invention to provide an electronic ballast that clamps the voltage across the main switch and eliminates voltage spikes due to the leakage inductance of the transformer.
It is another aspect of the present invention to provide an electronic ballast that recycles the leakage energy of the transformer to improve the power consumption efficiency of the circuit.
These and other aspects of the present invention are achieved by providing an electronic ballast that incorporates a clamping capacitor therein in such a manner that the overall circuit eliminates voltage spikes across the switches and recycles the transient leakage inductances of the transformer.
Thus, the invention comprises an electronic ballast that includes a DC power source and a transformer comprising first, second and third windings. The first, second and third windings being inductively coupled, and the third winding is connected in parallel with a load. The ballast also includes first and second circuit pathways connected in parallel. The first circuit pathway comprises a first switch connected in series with the first winding, and the second circuit pathway comprises the second winding connected in series with a second switch. The DC power source is connected in parallel with the first and second circuit paths to provide an input voltage source. A capacitor connects the point between the first switch and first winding of the first circuit pathway with the point between the second switch and the second winding of the second circuit pathway.
The first and second switches may be transistors, for example, metal-oxide semiconductor field-effect transistors. The first switch and the second switch are electrically connected to a digital controller or some other switching source that provides switching signals to the first and second switches. The first and second switches are switched off and on in a repeating cycle, the first switch switched on while the second switch is switched off and the first switch switched off while the second switch is switched on. When the first switch is switched off, a current in the first winding provides a charging of the capacitor. Likewise, when the second switch is switched off, a current in the second winding provides a charging of the capacitor. The voltage across the first and second switches is substantially constant over the interval that the respective switch is turned off.